Hitherto, not only AC power transmission, but also DC power transmission has been executed as electric power supply. The DC power transmission does not involve problems occurring in AC power transmission, such as an AC loss of a dielectric loss, a conductor loss caused by the skin effect, etc., and a large current flowing at the accident time of a short circuit, etc., for example, and large-capacity power can be transmitted. There are two systems of monopolar power transmission (monopole power transmission) and bipolar power transmission (bipole power transmission) for the DC power transmission.
In the monopolar power transmission, a power cable 100g as a going line and a power cable 100r as a return line are connected by leads 104 and 105 through DC/AC converters 102 and 103, forming a closed circuit, as shown in FIG. 11 (A). An AC system of an AC power supply 101 and a load using transmitted power is connected to the converters 102 and 103. In the bipolar power transmission, a power cable 100p as a positive pole line, a power cable 100m as a negative pole line, and a power cable 100n as a neutral line are provided in parallel and are connected by leads 104p, 104m, 104n, 105p, 105m, and 105n through DC/AC converters 102p, 102m, 103p, and 103m, forming a closed circuit, as shown in FIG. 11 (B). An AC system of AC power supplies 101 and loads is connected to the converters 102p, 102m, 103p, and 103m. 
The power cable used with such DC power transmission generally is a solid cable or an oil-immersed paper cable of an OF cable, etc. In addition to the normal conducting cable mentioned above, a superconducting cable for DC power transmission is proposed (for example, patent document 1).
On the other hand, to operate a power cable line, electric power machines such as various sensors and a monitoring device are used. With the superconducting cable, in addition to them, machines such as an auxiliary refrigerator, an auxiliary pump, and an auxiliary vacuum pump are also used.
Patent document 1: JP-A-2003-249130